The present invention relates to an improvement of signal integrity of analog signals based on the output of a delta-sigma modulator into which samples of the analog signal to be generated are fed.
It is known from prior art to use a delta-sigma modulator followed by a driver circuit and a filter and thus forming a delta-sigma digital-to-analog converter. Due to noise and circuit non-idealities the analog signal generated this way will always deviate from the ideal analog signal.
The filter following the delta-sigma modulator averages the bit stream of the mentioned driver and for an ideal signal integrity the contribution of each “high”-bit of the bit stream to the output signal must be equal, and the contribution of each “low”-bit of the bit stream to the output signal must be equal. However, memory effects in the driver circuit, known as inter symbol interference (ISI), affect—dependent upon history of previous bits—the pulse form of a bit and thus the contribution of this bit to the averaging process by the mentioned filter. Settling time, slew-rate limitations, unequal rise-/fall-times and ringing are contributing to the inter symbol interference.
The present invention relates also to improvements in the evaluation of a Device Under Test (DUT) in response to an input signal provided by an Automated Test Equipment (ATE) to said DUT, said input signal having improved integrity according to the inventive method and system.
Integrated Circuits (IC) generally need to be tested to assure proper operation. This—in particular—is required during IC development and manufacturing. In the latter case, the ICs are usually tested before final application. During test, the IC, as Device Under Test (DUT), is exposed to various types of stimulus signals, and its responses are measured, processed and usually compared to an expected response of a good device. Automated Test Equipments (ATE) usually perform these tasks according to a device-specific test program. Examples for ATE are the Agilent 83000 and 93000 families of Semiconductor Test Systems of Agilent Technologies Details are also disclosed e.g. in EP-A-859318, EP-A-864977, EP-A-886214, EP-A-882991, EP-A-1092983, US-A-5,499,248, US-A-5,453,995.
The present invention also relates to general test equipment as well as all types of analog signal generation.
Improving signal integrity by higher sampling rates or increasing the resolution of delta-sigma digital-to-analog converters is possible, however the known issue of inter symbol interference is not addressed by this kind of modifications.